This invention relates to a novel marine propulsion device which is suitable for use in conjunction with boats such as pleasure boats including yachts and motor boats. More particularly, the present invention relates to a novel marine propulsion device of the type comprising an engine which is mounted at a stern portion of boat, a propeller shaft which extends outwardly from the hull at a stern part along a rearwardly and downwardly inclined direction and which is equipped at the terminal end thereof with a propeller, clutch mechanism for transmitting power from said engine toward said propeller shaft selectively into the forward direction or backward direction of boat propulsion, and gearing means for connecting clutch output shaft of said clutch mechanism drivingly to said propeller shaft. The gearing means set forth above includes a transmission shaft which is aligned coaxially with the propeller shaft and is fixedly coupled to such propeller shaft.
A typical example of marine propulsion devices of the type set forth above is devices of so-called V-drive system. In this V-drive system, engine is mounted in the hull at a stern portion in a not inclined posture and with the output end being directed toward the stem of boat. On the front of lower portion of such engine is disposed a clutch means. Power transmission path is turned from a horizontal direction to a rearwardly and downwardly inclined direction by gearing means which is disposed on the front of the clutch means. Marine propulsion device of this drive system is called V-drive system because horizontal transmission path from the engine to the output end of clutch means is followed by inclined transmission path as from the gearing means so that power transmission path from the engine to propeller is V-letter shaped as a whole. As compared to another drive system such as the one in which the engine is mounted in a downwardly inclined posture with the output end of such engine being directed toward the stern of boat and in which power transmission path as from such engine to propeller is made straight along a rearwardly and downwardly inclined direction, the V-drive system provides a distinct advantage that an enlarged dwelling space is afforded in a pleasure boat or the like. This is because the V-letter shaped power transmission path of the V-drive system permits placing the engine nearer to the stern of boat as compared to another drive system and because the engine mounted in a not inclined posture occupies a minimum height as a whole. Enlargement of dwelling space is very important in a pleasure boat.
In employing V-drive or similar drive system having a transmission path which includes a bend or turn of direction, there is a problem how the bend or turn of transmission path is provided while insuring a smooth power transmission. The conventional way for solving this problem is the use of bevel gears in that a pair of mutually meshed bevel gears are fixedly provided on the output shaft of clutch means and a transmission shaft coupled to a propeller shaft. This conventional way has a disadvantage in that gearing means comprising the pair of bevel gears is relatively large in scale. The conventional way further has a serious disadvantage that manufacturing of such bevel gears is very difficult and requires a large cost. That is, the angle of downward inclination of the propeller shaft is about fifteen degrees (15.degree.) and bevel gears having a large cone distance of about 330 millimeters are required for bending or turning the power transmission path by such small angle. Manufacturing of bevel gears having such large cone distance requires a very large manufacturing machine, which makes such manufacturing difficult and very expensive.